Why could Luke sense the good in Darth Vader, but Obi-wan could not?

During their final duel on the Death Star, why could Obi-Wan not sense the good in Darth Vader, the way Luke later did?

In Episode 6, Luke says to Vader:

I can feel the good in you.

Luke's senses were not as deep as Obi-Wan's, who had years of training and knew Anakin inside and out. So he should have sensed less.